Questions regarding cascade control in DCS systems

By cascading the liquid level control to the flow rate of the flow meter at the outlet of the bottom liquid transfer pump, and then using the flow meter to adjust the valve position of the pump’s outlet valve, it is possible to achieve more precise and stable liquid level control. Firstly, the level of the liquid in the bottom of the distillation tower directly affects the inlet and outlet flow rates of the pump used to transfer this liquid. Using flow control as a feedback signal can more directly reflect changes in liquid level. Secondly, changes in liquid level lead to changes in inlet and outlet flow rates. By changing the setting value of the flow meter, the signal output by the flow meter can be adjusted, thereby enabling control of the valve position of the pump outlet control valve. In this way, the valve position can be adjusted in a timely manner according to changes in actual flow rate, thereby achieving stable liquid levels. If the liquid level is directly fed into a control valve to regulate its position, there may be certain delays and instabilities. Because changes in liquid level need to be fed back into the level control system through control valves, and the response speed and accuracy of these control valves are limited. In contrast, changes in flow rate are more easily detected by the flow meter and transmitted to the control system, allowing for more precise control of the liquid level. Overall, cascading the liquid level control with flow control enables more precise and stable liquid level regulation, reducing system latency and instability. .

Use it in whatever way works best; sometimes too complex a design can end up being less effective. If this cascade control system doesn’t work well, you can try using a single-loop level control system instead

Because of the large lag and slow response in liquid level changes, flow rate responds quickly to disturbances. Controlling the flow rate in order to regulate the liquid level has a predictive control effect, reducing the impact of disturbances on the primary variable and thereby making it more stable. This is a prominent feature of cascade control.

This level control system can only be connected in cascade to a flow meter for control; there is no control valve that directly regulates the level, so manual control has been used throughout. At present, the PID parameters have not been properly tuned, and implementing cascade control would cause excessive fluctuations in the system

This post was last edited by Agelxx on 2023-10-10 07:47. Didn’t your question refer to level cascade flow control? I think the large fluctuations have no direct relation to cascade control. First, verify whether the flow meter and control valve are working properly, and whether the flow single-loop system can be set to automatic mode Also, your statement that \"why not directly use the liquid level to control the valve position via a cascade control system in order to regulate the liquid level\" is incorrect. Cascade control involves using two variables to control one output; in other words, two PID modules are connected in series to ultimately control a control valve. Using just one liquid level variable to control the control valve in order to adjust the liquid level constitutes a simple single-loop control system, and it cannot be referred to as a cascade control system for the liquid level. Are you perhaps an undercover agent from the Instrument Control field? :lol

Try single-loop control; take some time to adjust the PID

I work in the process area, so I’m not very familiar with the system side. I asked our instrumentation engineer, who said that the instruments are only responsible for carrying out the functions required, while the actual design is determined by the process requirements. It is precisely because this cascade control cannot keep up with the liquid level, and is not as accurate as other liquid level control systems, that I am seeking an answer